Thermodynamic stability and guest distribution of CH4/N2/CO2 mixed hydrates for methane hydrate production using N2/CO2 injection

In this study, thermodynamic stability and cage occupation behavior in the CH4 - CO2 replacement, which occurs in natural gas hydrate reservoirs by injecting flue gas, were investigated with a primary focus on phase equilibria and composition analysis. The phase equilibria of CH4/N2/CO2 mixed hydrates with various compositions were measured to determine the thermodynamic stability of gas hydrate deposits replaced by N2/CO2 gas mixtures. The fractional experimental pressure differences (Δp/p) with respect to the CSMGem predictions were found to range from −0.11 to −0.02. The composition analysis for various feed gas mixtures with a fixed N2/CO2 ratio (4.0) shows that CO2 is enriched in the hydrate phase, and the N2/CO2 ratio in the hydrate phase is independent of the feed CH4 fractions. Moreover, 13C NMR measurements indicate that N2 molecules preferentially occupy the small 512 cages of sI hydrates while the CO2 molecules preferentially occupy the large 51262 cages, resulting in an almost constant area ratio of CH4 molecules in the large to small cages of the CH4/N2/CO2 mixed hydrates. The overall experimental results provide a better understanding of stability conditions and guest distributions in natural gas hydrate deposits during CH4 - flue gas replacement.